In telephone circuits, for reasons of line economics and easy replaceability, a two-wire configuration is generally adopted for a subscriber line that is connected to a subscriber's telephone set. The two-wire configuration constitutes a configuration wherein a single pair of wires is provided to carry signals in both directions. However, the line to which the subscriber line is connected utilizes a four-wire configuration which provides separate paths for each direction that is adopted.
In the conventional system, the subscriber lines differ in type and length. Therefore, each subscriber line has its own associated impedance and it is therefore difficult to match the hybrid coil perfectly. As a result, the hybrid coil not only passes the signal received from the distant party via the four-wire line to the local party via the subscriber line, but also acts as an echo path. This echo path allows the received signal to leak over to the transmitting side, where it becomes an echo signal that degrades communication quality. To cancel the echo signal, an echo canceller is typically deployed which is connected before the four-wire/two-wire conversion point.
A typical echo canceller may utilize data converters to convert between the analog and digital domains, such that a received analog signal can be converted to the digital domain, processed in the digital domain, and then converted back to the analog domain on both the transmit and receive paths. An adaptive filter in the digital domain is connected between the two paths, with the input of the adaptive filter connected to the received path and the output of the adaptive filter providing an input to a subtraction circuit for adjusting the transmitted value.
When the input signal is received from the distant party, it is sampled by an A/D converter to generate a discrete value at a sample time k, and thus convert it to a digital received signal. This digital received signal is converted by the D/A converter on the output of the receive path to an analog receive signal and then sent to the hybrid and, subsequently, the subscriber line to the local party. If the impedances are not matched, however, the analog receive signal follows an echo path and reaches the transmitting side as an echo signal. The echo signal is sampled by another A/D converter on the subscriber line side at a time k to generate a discrete value at time k, and is thus converted to a digital echo signal, which is fed to the subtractor.
The adaptive filter is operable to estimate the characteristics of the echo path and, from the estimated characteristics of the digital received signal, generate a simulated echo signal which is fed to the subtractor. The subtractor subtracts the simulated echo signal or echo signal from the digital echo signal and generates the difference as a residual signal. The adaptive filter is operable to cancel the echo signal so as to force the residual signal to converge as close as possible to zero.
The above description was directed toward "network echo", the other type being "acoustic echo", double talk presenting a problem to both types. In this type, the adaptive estimation function of the adaptive filter operates normally in the single-talk state, but in the double-talk state in which there is preferentially a transmitted signal from the local party, i.e., due to the party speaking, the estimation function of the adaptive filter is inaccurate, in that it may not provide the appropriate estimation function due to interference from the additional transmitted signal from the local party. Accordingly, echo cancellation devices which employ adaptive filters for estimating a room's response typically include a "double-talk" detection device which monitors the microphone signal to determine when a person is speaking into the microphone. One such detector, described in D. L. Duttweiler, "A Twelve Channel Digital Echo Canceller", IEEE Trans. On Comm., Volcom-26, No. 5, May 1978, declares double-talk when a sample of the microphone signal is greater than or equal to one-half the largest sample of the loudspeaker signal within the last N samples, where N is a constant equal to the maximum delay between the loudspeaker and the microphone. If someone is speaking into the microphone, the energy of the microphone signal is typically at least half that of the loudspeaker signal. Accordingly, the above described double talk detector properly concludes that someone is speaking into the microphone and disables the adaptive filter from adjusting its taps.
If the loudspeaker and microphone are far apart from each other, the microphone includes little or no acoustic feedback from the loudspeaker. Further, when someone is speaking softly into the microphone, the energy of the soft voice component of the microphone signal is not alone greater than half the energy of the loudspeaker signal. Accordingly, the above described double talk detector falsely concludes that no one is speaking into the microphone and therefore enables the adaptive filter to adjust its taps. The filter accordingly begins adjusting the taps in an effort to reduce the echo received at the microphone to zero. Thus, by falsely concluding that no one is speaking into the microphone, the device begins to cut off the voice of the person speaking into the microphone.
If the loudspeaker is placed close to the microphone, the energy of the microphone signal may exceed half the energy of the loudspeaker signal regardless of whether someone is speaking into the microphone. For example, if the room includes ambient background noise such as generated by a fan, the microphone picks up this sound and adds it to the substantial acoustic feedback caused by the close proximity of the microphone and loudspeaker. Accordingly, the energy of the microphone signal may exceed the half of the energy of the loudspeaker signal even when the loudspeaker is the only source of speech in the room. In this case, the above described double talk detector falsely concludes that someone is always speaking into the microphone and therefore permanently disables the adaptive filter from adjusting its taps.
In the above described operation, the system operated in a "full-duplex" mode. However, in some situations, the near-end and far-end have echo paths that result in an unstable operation, causing acoustic howling. One simple way of preventing a far-end user from hearing echos is to turn off the microphone at the near-end while the far-end user is talking, this referred to as half-duplex operation. In general, the prior half-duplex systems have involved a comparison of the signal power at the near-end either to a threshold, or to the signal power at the far end. This incurs some problems in the form of a dependence on relative power levels and a susceptibility to noise.
Another technique for curing acoustic howling, due to the loop becoming unstable due to the loop-gain being too high, is to utilize what is referred to as an "echo suppressor". This is facilitated by inserting one attenuator in the transmit path and another attenuator in the receive path. If either attenuator is on at any time, there is sufficient loop-gain reduction to prevent acoustic howling. The control logic for the attenuators would then be designed such that at least one attenuator is on at any particular time. In situations where an echo exists, the attenuator that is present in the path from the near-end microphone to the far-end is designed to have at least enough attenuation to prevent echo from being objectional to the far-end user. The control logic would then be designed so that this attenuator is always on whenever potentially objectional echo is present.
One disadvantage to current echo suppressors is that they do not adequately accommodate full-duplex communication. Only the far-end or the near-end can talk at any particular time; both cannot talk and be heard simultaneously. The result is that the user at one end cannot interrupt the other while the other is talking. This is a serious limitation that can make communication unnatural and unpleasant. However, if full-duplex communication is accommodated, some form of echo cancellation must be utilized. With echo cancellation, echo is removed from the signal received at the near-end without attenuating the speech originating at the near-end. A serious drawback to echo cancellation, however, is that it typically requires an adaptive filter, which is much more expensive to implement than the attenuation and control in an echo suppressor.